Optical-fibre passively mode locked laser generator with nonlinear polarisation switching

Abstract: A passively-mode-locked laser generator comprising an active optical fibre and fibre pumping means adapted to cause the generation of an emission signal in the fibre, a polarization control means, signal extraction means and at least one optical component with transmissiveness depending on polarization, all inserted in an optical fibre path that, for a pumping energy lower than a predetermined value corresponding to a linear propagation condition of the signal, gives rise to a polarization rotation of the emission signal through an angle causing the extinction of the signal itself within said optical component with transmissiveness depending on polarization and, for a pumping energy of a higher power than said predetermined value, causes a polarization rotation of the emission signal through an angle corresponding to the transmission with low losses within said optical components of the only pulses of high peak power of the emission signal.

Description: Under many circumstances in the telecommunications field it is desirable to obtain pulsated laser emissions of a particularly limited duration. Pulses of the above kind having a duration δt < 100 ps, commonly referred to as ultrashort pulses, or "solitons" under guided propagation conditions, apply for example in the field of high speed digital telecommunications (up to 10 GBit/s), for making optical instruments, carrying out tests on semiconductor components and also remote measurements, in topography or in the atmospheric-radar field, for example. To this end, optical-fibre laser devices using erbium-doped optical fibres are known, in which an active modulating device of the electro-optical type is inserted in an optical path forming a laser cavity so as to enable propagation and amplification of the desired pulses alone. Such devices are commonly referred to as mode-locked devices of the active type, because the modulating device present therein and operated from the exterior acts on the modes generated in the laser, thus enabling amplification as far as a level supporting the laser emission in the selected modes alone. Devices of the above kind are for example described in OPTICS LETTERS, vol. 14, No. 22, November 15, 1989, pages 1269-1271 by J.D. Kafka, T. Baer and D.W. Hall, in ELECTRONICS LETTERS, vol. 26, No. 3, February 1, 1990, pages 216-217, by A. Takada and H. Miyazawa, and in PROCEEDINGS OF II TOPICAL MEETING ON OPTICAL AMPLIFIERS, Optical Society of America, 1991, Snowmass Village, Colorado, US, pages 116-119, by T. Pfeiffer and H. Schmuck (SEL Alcatel Research Centre). Also known are mode-locked pulse laser generators of the passive type comprising optical-fiber laser devices (a "figure-of-eight" laser) in which a nonlinear amplifier loop mirror (NALM) is employed which is connected to a unidirectional loop to make a laser in which mode-locking is generated in the absence of external drive means. In the above devices the nonlinearity of the amplifier loop, when the highest luminous intensities occur, causes a light deviation from one inlet fibre to the other at the unidirectional loop; in this manner the system exhibits lower losses at higher luminous intensities than at lower luminous intensities, so that only pulses of greater intensity are maintained; this condition forces the laser to operate in a pulsated manner. Devices of this kind are described in ELECTRONICS LETTERS; Vol. 27, No. 6, March 14, 1991, pages 542-543, by D. J. Richardson, R. I. Laming, D.N. Payne, V. Matsas, M. W. Phillips, in ELECTRONICS LETTERS, Vol. 27, No. 9, April 25, 1991, pages 730-732 by the same authors, and in ELECTRONICS LETTERS, Vol. 27, No. 6, March 14, 1991, pages 544-545, by I.N. Duling. Theoretical foundations concerning mode-locking both of the active and passive type are disclosed, for example, in WAVES AND FIELDS IN OPTOELECTRONICS, by Herman A. Haus, issued in 1984 by Prentice-Hall, Inc., Englewood Cliffs, New Jersey, pages 254-290. In ELECTRONICS LETTERS, Vol. 27, No. 3, January 31, 1991 pages 229-230, G. J. Cowle and D. N. Payne describe a loop laser device comprising a selective reflector or mirror, a directional coupler, a polarization controller, an erbium-doped optical fibre and an optoisolator, disposed so as to form a resonant cavity. Such a device is specifically provided for continuous operation in a longitudinal mono-mode. In accordance with the invention it has been found that a laser emission having very short pulses, i.e. < 100 ps, can be achieved with a fibre laser using an erbium-doped active optical fibre with a passive mode locking, by the use of nonlinear-polarization rotation devices within the resonant cavity, in the presence of a narrow-band reflector means responsive to polarization. In particular, it has been found that a laser system comprising polarization controlling members and a narrow-band reflector means responsive to polarization, in which said members are selected and disposed according to a particular configuration enables the pulsated operation of the assembly to be achieved and a continuous-emission operation to be stopped, and under such a condition enables said very short pulses to be generated in the absence of active modulator devices or saturable absorbing elements. Therefore, it is an object of the present invention an active-fibre passively-mode-locked laser generator comprising: an active optical fibre doped with a fluorescent dopant; a means for feeding luminous pumping energy to one end of the active fibre, said pumping energy being adapted to excite the active-fibre fluorescent dopant to a laser emission state, from which said dopant may decay to a base state with a luminous emission at a predetermined wavelength constituting an emission signal comprising pulses of different peak powers, of a value depending on the power of said pumping energy; a polarization control means adapted to orient the polarization of said emission signal in a predetermined plane; feedback means optically connected to the ends of said active fibre; at least one optical component with transmissiveness depending on polarization; extraction means for drawing said emission signal from said active fibre; in which at least the active fibre, polarization control means, feedback means and optical component with transmissiveness depending on polarization are components of an optical fibre path, in which said emission signal propagates and in which at least one portion of the emission signal present at one end of the active fibre is sent back to the interior of said fibre; characterized in that the components of said optical path have an orientation that, on the whole, gives rise to a rotation of the polarization of said emission signal which, in case of pumping energy having a power lower than a predetermined value causing a linear-condition propagation of said emission signal in the optical path, takes place through an angle corresponding to the extinction of the emission signal within said optical component with transmissiveness depending on polarization and to the consequent absence of emission signal in said extraction means, and, in case of pumping energy of higher power than said predetermined value causing a non-linear-condition propagation of said emission signal in the optical path, takes place through an angle corresponding to the transmission with low losses within said optical component for pulses of high peak power of the emission signal only, so giving rise to ultrashort pulses of the emission signal. In a particular embodiment of the active-fibre passively-mode-locked laser generator in accordance with the invention the means supplying luminous pumping energy comprises a laser emitter of a pumping wavelength, connected through a respective optical fibre to an inlet of a dichroic coupler having one outlet at which said pumping energy and emission signal are coupled, and having an inlet for said emission signal. Said predetermined value of the pumping power is included within an adjustment field of the emission power of the pumping laser emitter, and preferably in the range of 7 to 10 mW. In a preferred embodiment of the invention said polarization rotation means of said emission signal travelling through said active fibre consists of at least one optical-fibre turn, serially connected in said optical fibre path, with a lying plane of adjustable orientation relative to the remaining portion of the optical path itself. Alternatively, said polarization rotation means of said emission signal travelling through said active fibre consists of a liquid-crystal polarization controller or a planar-optics polarization controller. Preferably, said optical component having transmissiveness depending on polarization has a polarization selectivity greater than 10%. In an embodiment said optical component with transmissiveness depending on polarization is a Bragg grating reflector; alternatively, said optical component with transmissiveness depending on polarization is a polarizer. In a preferred embodiment the optical fibre path comprises an active fibre having one end connected to one inlet of a directional coupler which has a second inlet connected to a second end of the active fibre, one outlet connected to a selective reflector, with transmissiveness depending on polarization, and a second outlet constituting the extraction means of said emission signal, a unidirectional optoisolator being interposed between said first and second inlets of said directional coupler in series with said active fibre, said polarization control means for controlling and adjusting the polarization rotation being interposed in said optical fibre path at an intermediate position between one of said active-fibre ends and selective reflector, and the means for supplying pumping energy being serially interposed within said optical fibre path connected to one of said ends of said active fibre. In an alternative embodiment, the optical fibre path comprises an active fibre having one end connected to a wideband reflector and a second end connected to the inlet of the selective reflector, with transmissiveness depending on polarization, the polarization control means for controlling and adjusting the polarization rotation being inserted within the optical path at an intermediate position between the selective reflector and wideband reflector, the means for supplying pumping energy being serially interposed within said optical fibre path, and connected to one of said ends of said active fibre. According to a further alternative embodiment, the optical fibre path comprises an active fibre connected in a closed circuit comprising the polarization control means for controlling and adjusting the rotation of polarization, a polarizer, a unidirectional optoisolator, means for supplying pumping energy connected to one of said ends of said active fibre, and a directional coupler for extracting the emission signal. Preferentially, the fluorescent dopant of the active fibre is erbium. More preferentially, said means for supplying luminous pumping energy has an emission of 980 nm pumping wavelength and said predetermined wavelength of said emission signal is in the range of 1530 to 1560 nm. According to a further aspect, the present invention comprises a method of generating passively-mode-locked stimulated pulses in an optical-fibre laser generator comprising: an active optical fibre doped with a fluorescent dopant; a means for feeding luminous pumping energy to the active fibre, said pumping energy being adated to excite the fluorescent dopant to a laser emission state, from which said dopant may decay to a base state with a luminous emission to a predetermined wavelength constituting an emission signal a polarization control means adapted to orient the polarization of said emission signal in a predetermined plane; feedback means optically connected to the ends of said active fibre; at least one optical component with transmissiveness depending on polarization; extraction means for drawing said emission signal from said active fibre; in which at least the active fibre, polarization control means, feedback means and optical component with transmissiveness depending on polarization are components of an optical fibre path, in which said emission signal propagates and in which at least one portion of the emission signal present at one end of the active fibre is sent back to the interior of said fibre; characterized in that it comprises the steps of: i. adjusting said polarization control means to a configuration causing the absence of an emission signal coming out of said extraction means in a linear propagation condition of the emission signal within said optical path; ii. supplying said active fibre with a pumping power corresponding to the stimulated emission, in the active fibre of an emission signal having a luminous power higher than a predetermined value, corresponding to nonlinear luminous-propagation conditions of the emission signal in the optical path, while keeping the configuration of the polarization control means constant and so giving rise to ultrashort pulses of the emission signal. Preferably, the method of generating stimulated passively-mode-locked pulses in accordance with the invention comprises the steps of: i. supplying said active fibre with a pumping power corresponding to the stimulated emission, in the active fibre of an emission signal having a luminous power lower than a predetermined value, corresponding to linear luminous-propagation conditions of the emission signal in the optical path; ii. controlling the presence of said emission signal coming out of said extraction means; iii. adjusting said polarization control means in a configuration causing the absence of an emission signal coming out of said extraction means; iv. increasing the pumping power supplied to said active fibre as far as a second predetermined value is overcome, which value corresponds to nonlinear luminous-propagation conditions of the emission signal in the optical path in which the polarization of the emission signal is rotated at right angles to that in said linear propagation conditions, while keeping the configuration of the polarization control means constant. In particular the pumping power is adjustable between a predetermined minimum and maximum values, in which said minimum value is lower than a value corresponding to the stimulated emission of said active fibre propagating in a linear condition in said optical path and said maximum value is higher than a value corresponding to a stimulated emission in said active fibre corresponding to nonlinear conditions of luminous propagation of the emission signal in the optical path in which the polarization of the emission signal is rotated at right angles to that in said linear propagation conditions; preferentially, said predetermined minimum value of the pumping power being lower than or equal to 7 mW and said predetermined maximum value of the pumping power being greater than or equal to 10 mW. In a preferred embodiment said polarization control means consists of at least one optical fibre turn, serially connected in said optical fibre path, capable of spatial orientation, and said arrangement of the polarization control means itself in a configuration causing the absence of an emission signal from said signal extraction means is accomplished by varying the spatial orientation of said optical fibre turn or turns. More details will appear from the following description, with reference to the accompanying drawings, in which: Fig. 1 is a diagram of a passively-mode-locked laser device according to one embodiment of the invention; Fig. 2 is an enlarged perspective diagram of a selective Bragg grating reflector; Fig. 3 is a perspective diagram of a polarization selector; Fig. 4 is a diagrammatic view of the energy transitions of a three-level laser system; Fig. 5 is a diagram showing the luminous intensity emitted from the active fibre in the absence of nonlinearity effects; Fig. 6 is a diagram showing the luminious intensity emitted from the active fibre in a transient step on the occurrence of nonlinearity effects; Fig. 7 is a diagram of the luminous intensity emitted from the active fibre under normal conditions, in the presence of nonlinearity effects and pulsated emission; Fig. 8 is a diagram of a laser device of the invention, according to an alternative embodiment with respect to Fig. 1; Fig. 9 is a diagram of a laser device of the invention, according to a further alternative embodiment thereof. An experiment for producing a passively-mode-locked laser in accordance with the invention has been executed with the use of a device having the configuration shown in Fig. 1. The laser, generally denoted by 1, comprises an erbium-doped active fibre 2; connected to one end of the fibre 2 is the trailing fibre 3 of a dichroic coupler 4 having one leading fibre 5 connected to a pump laser 6 and a second leading fibre 7 connected to a directional coupler 8. The fibre 9 coming out of the directional coupler 8 is connected at its end opposite to the coupler, to a polarization-selective reflector 10. The end of the active fibre 2 on the other side from the dichroic coupler 4 is connected, through an optical fibre 11, to the directional coupler 8, in the same direction as the fibre 7; a control polarization element 12 and an optoisolator 13 are interposed along the fibre 11. The optical fibre 14 coming out of the directional coupler 8 constitutes the outgoing fibre of the laser, to which a receiving apparatus R has been connected through which the emissions of laser 1 have been analysed. Fibres 7, 2, 11 all together form a loop A that, in conjunction with the reflector 10, constitutes a loop laser structure. The overall length of the loop A was about 27 m. The features of the active fibre 2 of the experiment carried out are the following: fibre length 10 m fibre type Si/Ge Numerical aperture (n 1 2 - n 2 2 ) ½ NA = 0.19 Cutoff wavelength (LP11 cut-off) λ c = 900 nm Erbium content in the core 100 ppm The dichroic coupler 4 is a fused-fibre coupler formed with mono-mode fibres of both 980 and 1536 nm wavelength, with variation of the optical output power depending on a polarization < 0.2 dB; the used coupler has been expressly made of fibres having the features of the above indicated active fibre 2 (apart from the erbium content, erbium being absent). Dichroic couplers of the stated type are known and produced for example by GOULD Inc., Fibre Optic Division, Baymeadow Drive, Glem Burnie, M.D. (US), and SIFAM Ltd., Fibre Optic Division, Woodland Road Torquay Devon (GB). The limitation stated for sensitivity to polarization aims at avoiding that with couplers more sensitive to polarization, in the presence of thermal variations or mechanical stresses causing a polarization variation in the emission of the pump laser 11, emission instabilities should occur. The pump laser 6 is a laser of a type known as "Strained Quantum Well", having the following features: Emission wavelength λ p = 980 nm Maximum optical output power P u = 35 mW. The optical output power of the pump laser is adjustable from 0 to 35 mW by a low-noise feed circuit known per se. Lasers of the above type are for example produced by: DAVID SARNOFF RESEARCH CENTER, Washington Rd, Princeton, N.J. (US). The directional coupler 8 is a device adapted to divide an optical signal present at one of its inlets into predetermined proportions at the outlets thereof. The directional coupler employed in the described experiment is a commercial type having a 50/50 division ratio, for example a model 1550 POH 50/50 2x2 produced by the above mentioned company Gould Inc. The selective reflector 10 is a Bragg grating reflector made of optical fibre and has the following features: Reflectivity 35% Reflected wavelength λ r = 1531 nm Bandwidth 0.7 nm Polarization selectivity 25% The Bragg grating reflector substantially consists of a portion of optical fibre, shown in Fig. 2, the core 16 and cladding 17 of which have been partly removed over a length thereof, forming a surface 18 on which several parallel wavings 19 have been formed by a photochemical process, which are transverse to the longitudinal axis of the fibre and have a pitch correlated with the reflected wavelength Λ by the relation λ = 2 n e Λ, wherein n e is the real refraction index of the fibre mode. By "polarization selectivity" it is intended the percent ratio, for a reflecting component, between the reflectivity in the most unfavoured polarization plane (minimum reflectivity) and the reflectivity in the polarization plane perpendicular to the preceding one (maximum reflectivity). By "a component passed through by a luminous signal" it is intended the percent ratio between the luminous power passing through the component in the most favoured polarization plane (maximum transmissiveness) and the luminous power passing through the component in the polarization plane perpendicular thereto (minimum transmissiveness). Reflectors of the above type are available on the market from: UNITED TECHNOLOGIES PHOTONICS, Silver Lane, East Hartford, US. In the reflector used the width of the reflected band was 0.05 nm, at a 1536 nm wavelength λ corresponding to a grating pitch Λ of about 500 nm. The fibre coming out of the reflector 10 on the other side from the fibre 9, has been cut obliquely so as to be unreflecting. Fibres 11, 7, 9 and 14 are mono-mode optical fibres type 8/125 (in which 8 represents the core diameter and 125 the cladding diameter in the fibre, reproduced in µm), the core being doped with germanium and having a numerical aperture NA = 0.13; these fibres are available on the market. The polarization control element 12 shown in Fig. 3 consists of three optical-fibre coils 20, 21, 22, each formed with several turns disposed in succession and supported according to an adjustable orientation so that they can be oriented in different planes through rotation relative to a common alignment axis. In the embodiment shown coils consisting of 4, 2 and 4 turns respectively have been used, the diameter of said coils being 6.32 cm and the coils being made of an optical fibre having the same features as the above described optical fibres 11, 7, 9 and 14. The turn orientation has been determined experimentally relative to the overall transmissive features of the loop. Devices of the above type can be made expressly using the fibres of the laser loop; otherwise commercially available devices can be employed, for example produced by GEC MARCONI MATERIALS TECHNOLOGY Ltd, Caswell, Towcester Northants NN12 8EQ (Great Britain). While the polarization control element is preferably made of adjustable turns, as above described, for specific application features other types of polarization controllers may be also used, such as liquid-crystal controllers, available from ADVANCED OPTRONICS Inc., 2121-B Ringwood Ave., San Jose, CA, US, or planar optics controllers or the like, by which the desired polarization adjustment of the signal circulating in the laser cavity may be achieved. The optoisolator 13 is a polarization control optoisolator of a type independent of the polarization of the transmission signal having an isolation greater than 35 dB and a reflectivity lower than -50 dB. The isolator used is a model MDL I-15 PIPT-A S/N 1016 produced by ISOWAVE, 64 Harding Avenue, Dover, New Jersey, US. The receiving apparatus R consists of an auto-correlator, type FR-1 3KR, produced by FEMTO-CHROME INC., US. The described apparatus was tested as follows. The pump laser 6 was operated until emission of a power equal to about 8 mW, and at the receiver R the beginning of a laser emission at 1536 nm was detected, of a power slightly higher than the spontaneous emission level (about 15 µW). Then the polarization control element 12 was adjusted so as to completely switch off the observed laser action, and the absence of a receiving signal at the receiver R was ascertained. To this end turns 20, 21, 22 were oriented so that at the receiver R no signal should be detected at an optical pumping power approximately corresponding to 6-7 mW (that is slightly higher than the threshold power, in which the active fibre gain exceeds the losses thereof). The pump power was then increased to a maximum value of 35 mW, and after a transient of some tens of µs a stable condition of pulse emission at a constant frequency of 7.8 MHz and a duration of about 10 ps was observed; at the maximum power (35 mW) of the employed pump the detected emission power was equal to about 5 mW. The width δt of the emitted pulses was calculated as corresponding to about 50 ps. The observed phenomenon can be interpreted as follows. On operation of the pump laser 6 above the threshold power of 6-7 mW, the active fibre 2 begins emitting luminous signals following the spontaneous transition of erbium present therein from the laser-emitting energy level to the base state. As shown in the diagram of Fig. 4 relating to a fibre of the above type symbolically representing the available energy states for an erbium ion in solution in the fibre silicon matrix, the admission of luminous energy at the "pumping" wavelength λ p into the active fibre brings a certain number of Er 3+ ions present as the doping substance in the fibre glass matrix to an "excited" energy state 23, named "pumping" band, from which ions spontaneously decay to an energy level 24, forming the laser emitting level. In the laser emitting level 24, Er 3+ ions can stand for a relatively long period before undergoing a spontaneous transition to the base level 25. As known, whereas transition from band 23 to level 24 is associated with an emission of the thermal type which is dispersed outside the fibre (phononic radiation), the transition from level 24 to the base level 25 is associated with a luminous emission having a wavelength λ s corresponding to the energy value of the laser emitting level 24. The spontaneous decay of erbium atoms from the laser level to the base level generates then a luminious emission at the wavelength λ s . If the active fibre at the laser emission level contains a number of ions higher than at the base level, photons emitted by spontaneous decay are more likely, while travelling through the fibre, to encounter ions at the laser level rather than at the base level, by which ions they will be absorbed, and therefore they cause a stimulated ion transition from the laser level to the base level, before their spontaneous decay, thereby giving rise to a cascade phenomenon producing at the exit of the active fibre a signal consistent with the wavelength λ s . On the whole, the transitions generated within the active "pumped" fibre produce several signal components at the same wavelength λ s , which transitions by interference produce an emission having several peaks S1, S2, S3, Sn with different polarizations and intensities, as qualitatively shown in the diagram in Fig. 5. The signal coming out of the active fibre 2 travels through the loop formed by the fibre 11 and reaches the directional coupler 8, a fraction thereof (50% with the described apparatus) being sent to the selective reflector 10. The reflected signal comes back to the directional coupler 8 and the signal fraction which is routed to the fibre 7 is fed to the active fibre 2 again; the photons at the wavelength λ s encounter new erbium atoms excited to the laser emission level by the administered pumping energy and cause the decay thereof, with the associated consistent emission and further amplification of the signal itself, thereby giving rise to the laser emission. The signal fraction routed to the fibre 11 in an opposite direction relative to that allowed by the isolator 13, is stopped by said isolator. The selective reflector 10 is a selective polarization device, which means that the reflection of a signal varies depending on the polarization state of same; in particular, the selective reflector used had a 25 dB reduction in the reflected power, with a luminous signal having a polarization perpendicular to the optimal one. The polarization selector 12 by suitable rotations of the lying planes of the turns 20, 21, 22 to be determined experimentally, enables the polarization plane of the signal travelling through the fibre 11 and sent to the selective reflector 10 to be modified. The double refraction features of the fibres forming the laser loop A together with the geometrical configuration of the loop cause a polarization rotation of the light travelling through the loop itself, as described for example by G.P. Agarwall, NON LINEAR FIBER OPTICS, Academic Press, Inc., Harcourt Brace Ivanovich, Publishers, US. If the orientation of the polarization selector 12 is selected so that it may be able to compensate for the effect resulting from the double refraction of the loop fibres, in order to achieve an overall zero polarization rotation of a signal travelling through the loop, the reflected component from the selective reflector 10 after travelling through the whole loop comes back to the reflector 10 with the same starting polarization plane. In this manner the signal component can be reflected again and by subsequent passages through the active fibre 2 it is increasingly more amplified thereby generating a continuous laser emission at constant frequency. This operating condition substantially corresponds to that of the device disclosed in ELECTRONICS LETTERS, Vol. 27, January 31, 1991, pages 229-230. According to the present invention, if the orientation of the polarization selectors is on the contrary such selected that it causes a 90° polarization rotation of the signal travelling through the loop, as a result, in the case of a low density signal (that is having a low pumping power) the signal component initially possessing a polarization state adapted to be reflected from the selective reflector 10, is reflected by said reflector and travels through the loop A again, therein undergoing a 90° polarization rotation and arriving again at the reflector 10 with a polarization plane in which no reflection occurs. Said orientation condition of the polarization selector with a 90° polarization rotation in the travel path of the loop thus causes the complete absence of laser emission and for this reason it is excluded from the teachings of the above article. By "low density signal" it is meant a signal that does not considerably alter the refraction index of the fibre; by way of example a signal having said intensity corresponds to a power P ≤ ≈ 2mW . (The signal intensity is I = P/A f , where A f is the fibre area). By "low pumping power" it is intended a power slightly higher than the threshold power, that is higher than the power capable of triggering the laser effect, but insufficient to generate an intensity laser signal; by way of example, in the experimented configuration, this low pumping power was about 6-7 mW; generally the intensity of the laser signal emitted increases substantially proportionally with the pumping power. In accordance with the invention, however, it has been found that by increasing the pumping power fed to the active fibre 2 beyond the above minimum value, without modifying the preselected configuration of the polarization selector in order not to generate a laser emission of low pumping power, it is possible to obtain the emission of the so-called ultrashort pulses or "solitons", that is pulses having a lower duration than 100 ps, high power (approx. > 1 mW) and capable of propagating unaltered (that is without distortion) through mono-mode optical fibres. This phenomenon is deemed to be due to the occurrence of non-linear phenomena of luminous propagation in the fibre, as a result of the variation of the refraction index in the fibre depending on the luminous intensity of the signal passing through it. The variation in the refraction index of a transparent means depending on the luminous intensity of the signal passing through it is given by: Δn = n 2 E 2 = γI, wherein n 2 is reproduced in electrostatic units; E is the electric field in electrostatic units and γ is a constant reproduced in cm 2 /W. In each polarization plane one can write: n x = n o + α E x E y n y = n o + α' E x E y etc. As can be seen from the preceding equations, in the presence of low-intensity signals, that is electric fields of low value, the refraction indices stand substantially constant and of same value; the variation in the refraction index occurring with pulses of greater intensity, on the contrary, generates a different variation of the polarization plane on travelling through the loop A which variation may reach such a value to compensate for the 90° polarization rotation previously selected by the polarization selector 12. Such pulses are thus reflected from the selective reflector 10 and therefore give rise to a selective laser action, that is for the high-intensity pulses alone, following which the most intense pulses are amplified preferentially with respect to the weaker ones, as shown in Fig. 6 by way of example only (pulses S2, S3). The amplification of the most intense pulses within the active fibre 12 causes an important transition to the base level of the excited erbium atoms, thereby subtracting them to the amplification of the weaker pulses that are therefore progressively attenuated as far as they become zero. After a transient of some tens of µs, a stable operating condition is therefore established in which a train of pulses S repeating at a constant frequency is generated, as shown in Fig. 7, which differentiates from a pedestal emission P of intensity 5% lower than the maximum pulse intensity. It should be noted that the known systems of soliton emission (NALM) previously cited and based on the path switching effect within the unidirectional loop usually exhibit, to the applicant's knowledge, a pedestal emission of intensity 10% higher than the maximum intensity of the emitted pulses. The reduced value of the pedestal emission found in the laser of the invention is deemed to be due to a greater sensitivity to polarization of the polarization-selective reflector, and more generally of the optical component with transmissiveness depending on polarization, in the presence of the above described non-linear phenomena, relative to the path switching selectivity of a directional coupler. The pulse emission frequency that is the time gap Δt between one pulse and the other is correlated with the light travelling time in the loop, that is its linear course. To the ends of the present invention the isolator 13 and polarization selector 12 can be disposed at all events within the loop included between the fibre 7 inlet for connection with the direction coupler 8 and the selective reflector 10; the pump laser and related dichroic coupler 4 can be also disposed in connection with either end of the active fibre 2, provided that the pumping energy is supplied to the active fibre itself without important attenuations. A similar result to the ends of the invention is achieved with laser devices having different configurations, such as those shown in Figs. 8 and 9 in which corresponding elements have been allocated the same reference numerals as in Fig. 1. In the laser shown in Fig. 8 the configuration is of a linear type, the so-called steady-wave type, and a wideband reflecting element 26 is employed therein, which element together with the selective reflector 10 defines an optical fibre path B in which at least one portion of the signal emitted at one end of the active fibre is sent back to the opposite end thereof. The wideband reflector 26 may consist of a reflecting element in microoptics, that is a fibre loop closed upon itself by a directional coupler or the like. The outgoing fibre 14 in this case is connected to the trailing fibre of the selective reflector 10, thereby enabling extraction from the laser and emission of the signal fraction that is not reflected thereby. In this device as well, the emission of the active fibre 2 possesses a polarization state that is rotated by turns 12 and fibre 11, thereby causing an overall polarization rotation through 90° of the signal travelling over the optical path B, so that the reflected component from the selective reflector 10, after travelling through the whole optical path, comes back to the reflector 10 with such a polarization state that, in case of a low-intensity signal, that is having a low pumping power, the signal component possessing a polarization plane adapted to be reflected from the selective reflector 10 travels back along the optical path B and is rotated again through 90° so that it arrives at the reflector 10 again with a polarization plane in which no reflection occurs, thereby generating the complete absence of laser emission. In the same manner as above described with reference to Fig. 1, by employing a pumping power supplied to the active fibre 2 higher than the stated minimum value and keeping the features of the fibres and components present in the laser cavity to the polarization conditions corresponding to the absence of emissions at a low pump and signal power, the occurrence of non-linear phenomena of variation of the refraction index in the fibre depending on the luminous intensity of the signal travelling over it produces the selective amplification of the only high-intensity pulses, thereby giving rise to the desired "ultrashort" pulses of high power. In the linear laser device of Fig. 8 too, the mutual arrangement of the polarization selector 12 and active fibre 2, as well as the end connecting the active fibre to the dichroic coupler 4 can be modified without important variations in the achieved result. Fig. 9 shows a further embodiment in which the laser is in the form of a closed circuit so that the outgoing signal at one end of the fibre 2 is sent back to the opposite end of the fibre 2 itself through the fibre 11. In this arrangement, instead of the Bragg grating reflector 10 of Figs 1, 8, an element responsive to polarization is employed which consists of an optoisolator of a type responsive to polarization; isolators of this kind are commercially available, for example from the above mentioned producer; alternatively, as diagrammatically shown in the figure, an optoisolator non-responsive to polarization 13 may be employed in association with a polarizer 13' (the placement of which in the laser cavity may be independent of the position of the isolator 13). The wavelength selectivity is obtained through a filter 27 consisting for example of a fibre portion involving two cores optically coupled in the desired wavelength band (the features of which are well known to a person of ordinary skill in the art) by which only a restricted wavelength band is transmitted to the cavity.

Claims: An active-fibre passively-mode-locked laser generator (1) comprising: an active optical fibre (2) doped with a fluorescent dopant; a means (3,4,5,6) for feeding luminous pumping energy to one end of the active fibre, said pumping energy being adapted to excite the active-fibre fluorescent dopant to a laser emission state (24), from which said dopant may decay to a base state (25) with a luminous emission at a predetermined wavelength constituting an emission signal comprising pulses of different peak powers, of a value depending on the power of said pumping energy; a polarization control means (12;13') adapted to orient the polarization of said emission signal in a predetermined plane; feedback means (10,26) optically connected to the ends of said active fibre; at least one optical component (10) with transmissiveness depending on polarization; extraction means (8,14) for drawing said emission signal from said active fibre; in which at least the active fibre, polarization control means, feedback means and optical component with transmissiveness depending on polarization are components of an optical fibre path, in which said emission signal propagates and in which at least one portion of the emission signal present at one end of the active fibre is sent back to the interior of said fibre; characterized in that the components of said optical path have an orientation that, on the whole, gives rise to a rotation of the polarization of said emission signal which, in case of pumping energy having a power lower than a predetermined value causing a linear-condition propagation of said emission signal in the optical path, takes place through an angle corresponding to the extinction of the emission signal within said optical component with transmissiveness depending on polarization and to the consequent absence of emission signal in said extraction means, and, in case of pumping energy of higher power than said predetermined value causing a nonlinear-condition propagation of said emission signal in the optical path, takes place through an angle corresponding to a transmission with low losses within said optical component for pulses of high peak power of the emission signal only, so giving rise to ultrashort pulses of the emission signal. An active-fibre passively-mode-locked laser generator according to claim 1, characterized in that the means for supplying luminous pumping energy comprises a laser emitter (6) of a pumping wavelength, connected through a respective optical fibre (5) to an inlet of a dichroic couple (4) having an outlet at which said pumping energy and emission signal are coupled, and having an inlet for said emission signal. An active-fibre passively-mode-locked laser generator according to claim 1, characterized in that said predetermined value of the pumping power is included within an adjustment field of the emission power of the pumping laser emitter. An active-fibre passively-mode-locked laser generator according to claim 1, characterized in that said predetermined value of the pumping power is in the range of 7 to 10 mW. An active-fibre passively-mode-locked laser generator according to claim 1, characterized in that said polarization rotation means (12) of said emission signal travelling through said active fibre consists of at least one optical-fibre turn (20,21,22), serially connected in said optical fibre path, with a lying plane of adjustable orientation relative to the remaining portion of the optical path itself. An active-fibre passively-mode-locked laser generator according to claim 1, characterized in that said polarization rotation means (12) of said emission signal travelling through said active fibre consists of a liquid-crystal polarization controller. An active-fibre passively-mode-locked laser generator according to claim 1, characterized in that said polarization rotation means (12) of said emission signal travelling through said active fibre consists of a planar-optics polarization controller. An active-fibre passively-mode-locked laser generator according to claim 1, characterized in that said optical component having transmissiveness depending on polarization has a polarization selectivity greater than 10%. An active-fibre passively-mode-locked laser generator according to claim 8, characterized in that said optical component with transmissiveness depending on polarization is a Bragg grating reflector (10). An active-fibre passively-mode-locked laser generator according to claim 8, characterized in that said optical component with transmissiveness depending on polarization is a polarizer (13'). An active-fibre passively-mode-locked laser generator according to claim 1, characterized in that the optical fibre path (A) comprises an active fibre (2) having one end connected to one inlet of a directional coupler (8) which has a second inlet connected to a second end of the active fibre, one outlet connected to a selective reflector (10), with transmissiveness depending on polarization, and a second outlet constituting the extraction means (14) of said emission signal, a unidirectional optoisolator (13) being interposed between said first and second inlets of said directional coupler in series with said active fibre, said polarization control means (12) for controlling and adjusting the polarization rotation being interposed in said optical fibre path at an intermediate position between one of said active-fibre ends and said selective reflector, and the means (3,4,5,6) for supplying pumping energy being serially interposed within said optical fibre path, and connected to one of said ends of said active fibre. An active-fibre passively-mode-locked laser generator according to claim 1, characterized in that the optical fibre path (B) comprises an active fibre (2) having one end connected to a wideband reflector (26) and a second end connected to the inlet of a selective reflector (10), with transmissiveness depending on polarization, the polarization control means (12) for controlling and adjusting the polarization rotation being inserted within the optical path at an intermediate position between the selective reflector and wideband reflector, the means (4,5,6) for supplying pumping energy being serially interposed within said optical fibre path, and connected to one of said ends of said active fibre. An active-fibre passively-mode-locked laser generator according to claim 12, characterized in that the means (12) for controlling and adjusting the polarization rotation are inserted within the optical path, between the active fibre (2) and wideband reflector (26). An active-fibre passively-mode-locked laser generator according to claim 1, characterized in that the optical fibre path comprises an active fibre (2) connected in a closed circuit comprising the polarization control means (12) for controlling and adjusting the polarization rotation, a polarizer (13'), a unidirectional optoisolator (13), means (4,5,6) for supplying pumping energy connected to one of said ends of said active fibre, and a directional coupler (8) for extracting the emission signal. An active-fibre passively-mode-locked laser generator according to claim 1, characterized in that said fluorescent dopant of said active fibre is erbium. An active-fibre passively-mode-locked laser generator according to claim 15, characterized in that said means for supplying luminous pumping energy has an emission of 980 nm pumping wavelength. An active-fibre passively-mode-locked laser generator according to claim 15, characterized in that said predetermined wavelength of said emission signal is in the range of 1530 to 1560 nm. A method of generating passively-mode-locked stimulated pulses in an optical-fibre laser generator (1) comprising: an active optical fibre (2) doped with a fluorescent dopant; a means (3,4,5,6) for feeding luminous pumping energy to the active fibre, said pumping energy being adapted to excite the fluorescent dopant to a laser emission state (24), from which said dopant may decay to a base state (25) with a luminous emission at a predetermined wavelength constituting an emission signal; a polarization control means (12) adapted to orient the polarization of said emission signal in a predetermined plane; feedback means (10;26) optically connected to the ends of said active fibre; at least one optical component (10,13') with transmissiveness depending on polarization; extraction means (8,14) for drawing said emission signal from said active fibre; in which at least the active fibre, polarization control means, feedback means and optical component with transmissiveness depending on polarization are components of an optical fibre path, in which said emission signal propagates and in which at least one portion of the emission signal present at one end of the active fibre is sent back to the interior of said fibre; characterized in that it comprises the steps of: i. adjusting said polarization control means to a configuration causing the absence of an emission signal coming out of said extraction means in a linear propagation condition of the emission signal within said optical path; ii. supplying said active fibre with a pumping power corresponding to the stimulated emission in the active fibre of an emission signal having a luminous power higher than a predetermined value, corresponding to nonlinear luminous-propagation conditions of the emission signal in the optical path, while keeping the configuration of the polarization control means constant, and so giving rise to ultrashort pulses of the emission signal. A method of generating stimulated passively-mode-locked pulses according to claim 18, characterized in that it comprises the steps of: i. supplying said active fibre with a pumping power corresponding to the stimulated emission in the active fibre of an emission signal having a luminous power lower than a predetermined value, corresponding to linear luminous-propagation conditions of the emission signal in the optical path; ii. controlling the presence of said emission signal coming out of said extraction means; iii. adjusting said polarization control means in a configuration causing the absence of an emission signal coming out of said extraction means; iv. increasing the pumping power supplied to said active fibre as far as a second predetermined value is overcome, which value corresponds to nonlinear luminous-propagation conditions of the emission signal in the optical path in which the polarization of the emission signal is rotated at right angles to that in said linear propagation conditions, while keeping the configuration of the polarization control means constant. A method of generating stimulated passively-mode-locked pulses according to claim 19, characterized in that the pumping power is adjustable between predetermined minimum and maximum values, in which said minimum value is lower than a value corresponding to the stimulated emission of said active fibre propagating in a linear condition in said optical path and said maximum value is higher than a value corresponding to a stimulated emission in said active fibre corresponding to nonlinear conditions of luminous propagation of the emission signal in the optical path in which the polarization of the emission signal is rotated at right angles to that in said linear propagation conditions. A method of generating stimulated passively-mode-locked pulses according to claim 20, characterized in that said predetermined minimum value of the pumping power is lower than or equal to 7 mW. A method of generating stimulated passively-mode-locked pulses according to claim 20, characterized in that said predetermined maximum value of the pumping power is greater than or equal to 10 mW. A method of generating stimulated passively-mode-locked pulses according to claim 18, characterized in that said polarization control means consists of at least one optical-fibre turn (20,21,22) capable of spatial orientation, serially connected in said optical fibre path, and said arrangement of the polarization control means itself in a configuration causing the absence of an emission signal from said signal extraction means is accomplished by varying the spatial orientation of said optical-fibre turn or turns.

Inventors: FONTANA FLAVIO, FONTANA, FLAVIO
Applicants: PIRELLI CAVI SPA, PIRELLI CAVI S.P.A.
Application year: 1994
Main classification: H01S3
IPC codes: G02F1, H01S3, H04B10